1. Technical Field
The present invention relates to a coaxial cable comprising a central conductor, insulation, an outer conductor, and a jacket, and a method for manufacturing the same.
2. Related Background Art
Coaxial cables used in small communication devices, electronic equipment, medical equipment, and the like are typically formed as thin cables having an outside diameter of 0.5 mm or less, and there is a demand for further diameter reduction. The diameter of the central conductor, the thickness of the insulation layer, and the like are being made smaller in response to the demand for diameter reduction, but there is also a demand for better mechanical strength and bending resistance from the standpoint of ensuring reliability. With the increasing signal transmission speeds of recent years, there is also a demand for a coaxial cable having minimal signal attenuation. Reducing signal attenuation necessitates a reduction in the dielectric constant of the insulation that surrounds the central conductor.
JP2007-172928A (Patent Document 1) describes a coaxial cable in which the central conductor is obtained by twisting thin conductors (wires) together, and the silver content and the heat treatment of the central conductor are optimized in order to minimize the resulting reduction in the dielectric constant and tensile strength. The central conductor of this coaxial cable is a copper alloy containing 1 to 3 wt % silver, with the balance being substantially copper. The central conductor is obtained by twisting together seven wires having a diameter of 0.010 mm to 0.025 mm, and has a tensile strength of 850 MPa or greater, electrical resistivity of 85% or greater relative to a standard annealed copper conductor, and an insulation thickness of 0.07 mm or less.
JP2007-169687A (Patent Document 2) describes a coaxial cable which has the aforementioned central conductor and foam insulation such that the insulation has a reduced thickness and a reduced diameter and the cable maintains an electrostatic capacitance at a predetermined level or below.
JP2007-242264A (Patent Document 3) describes a coaxial cable in which the insulation surrounding the central conductor is formed as a non-foam solid layer, and in which electrical and mechanical characteristics are improved by creating voids between the surface of the central conductor and the insulation. Creating a spiral ridge on the central conductor is described as the method of creating the voids. Specifically, it is disclosed that the cable is formed from a double-twisted wire obtained by twisting together two or three twisted wires in which two or three conductor wires are themselves twisted together.